1. Field of the Invention
The present invention is directed to a spacerless double bottom apparatus for a metal storage tank and a method of installation thereof. In particular, the present invention is directed to a spacerless double bottom apparatus that occupies a minimum of space in a metal storage tank.
2. Prior Art
In large storage tanks, such as those used for fluid hydrocarbon including oil and gas, the metal bottom of the storage tank may be subject to corrosion over time. The bottom may corrode for a number of service conditions and may corrode from the inside or from the outside.
It is possible to completely remove the corroded bottom and replace it with another replacement bottom although this is time consuming, expensive and does not address possible underside corrosion. Rather than replace the bottom of the tank, a practice has been developed in servicing of large storage tanks to add a new bottom parallel to and spaced from the existing bottom. In one existing practice, new bottom plates are added and welded to the sidewalls after inserting approximately four inches of sand on top of the old bottom. While this practice works to create a fluid-tight storage tank container, four to six vertical inches of the storage tank are lost in the process. Additionally, in the event that a leak develops through the new, upper bottom, while the sand may absorb some of the fluid, it will be difficult to determine when a leak has developed. In another existing practice, a second metal bottom is placed on top of a rebar grid inserted on top of the original bottom plate. The rebar grid acts as a spacer and allows fluid flow.
In another existing practice, a plastic liner is placed over the corroded bottom. Any welding near the plastic liner, however, will damage the liner.
Industry standards, such as API (American Petroleum Institute) 650 and 653, require each tank to have a nozzle or nozzles with reinforcing around the nozzle a certain distance from the tank bottom due to weld spacing limitations. If the secondary tank bottom is spaced too far from the original bottom, the nozzle will have to be relocated.
Other prior attempts to address these issues include Ershig (U.S. Pat. No. 4,871,081) which discloses a multi-layer floor for primary and secondary containment having: a lower floor 4 which includes a plastic sheet 6, a fiberglass reinforced plastic exterior 8, and an interior vertically oriented grating 10; and an upper floor 24 which includes an upper element 26, a lower element 28 and interior grating 30. Ershig is directed to a new double wall tank system using sandwich panels.
Lasson (U.S. Pat. No. 5,002,195) discloses a double-walled tank bottom including an outer shell bottom 12 and an inner shell bottom 14 spaced therefrom with a formed plastic sheet 60 in the gap 71. The plastic sheet has bosses 61 and depressions 59 and spaced support steel balls in selected depressions. Lasson requires rigid support on both sides of a liner which would not retrofit well where the existing bottom is deformed.
Henneck et al. (U.S. Pat. No. 5,269,173) discloses a bitumen layer applied on top of an outer tank bottom 10 covered by polyethylene film. A liquid permeable layer 16, such as drainage asphalt, is applied on top of the film with sensor cable 18 therein. An inner tank floor 28 is welded above the permeable layer 16.
Bachmann (U.S. Pat. No. 5,269,436) discloses a double walled tank with an outer wall and an inner wall. The inner wall is formed of a laminate comprising an embossed aluminum foil sheet 25 and a plastic layer 27. Bachmann utilizes foil and glass with epoxy which is not compatible with the heat of welding.
Skogman (U.S. Pat. No. 5,522,340) discloses a double walled vessel such as a tank including a first wall 32 spaced apart from a second outer wall 33 with an intermediate single woven member 34 therebetween. The woven member 34 has a plurality of longitudinally extending cylindrical members 36 positioned parallel to each other and a plurality of fibers 38 woven perpendicularly to the cylindrical members. Skogman is directed to a non-metallic tank.
Coates (U.S. Pat. No. 6,206,226) discloses a series of fiber reinforced plastic panels joined together by pop rivets to form an inner wall with an open grid of high density polyethylene between the inner plastic panels and the outer wall. Coates does not provide a metal retrofit bottom.
Piehler (U.S. Pat. No. 6,431,387) discloses in FIG. 3 a lining system with a plastic foil placed on a tank bottom 1 and a plastic grid and foil resting thereon.
Nevertheless, there remains a need to provide a double bottom apparatus for a large storage tank which is extremely thin and takes up a minimum amount of volumetric space in the storage tank.
There also remains a need to provide a spacerless double bottom apparatus for a large storage tank which includes a fluid tight sealed compartment for secondary containment which will contain any future leaks.
There also remains a need to provide a spacerless double bottom apparatus having a fluid tight sealed secondary containment compartment which may be negatively or positively pressurized.
There also remains a need to provide a spacerless double bottom apparatus which utilizes a thin sheet of plastic liner and, at the same time, utilizes an upper metal bottom that will be welded to the existing sidewalls of a storage tank without damage to the plastic liner.
There also remains a need to provide a spacerless double bottom apparatus for field retrofitting to an existing storage tank which may be simply installed.
There also remains a need to provide a spacerless double bottom apparatus which utilizes a plastic liner, a plastic grid to permit fluid flow, and a metal replacement bottom which minimizes any need for overhead welding.
There also remains a need to purge a sealed interstitial space with an inert gas or corrosion inhibitor to prevent corrosion.
There also remains a need to install a secondary containment liner in a tank storing a heated product without melting the liner.
There also remains a need to install a third bottom without greatly reducing capacity.
There also remains a need to create a closed interstitial space so as to be able to create a positive pressure differential for tracer gas (such as helium) detection of leaks.
There also remains a need to install a corrosion probe in the interstitial space to monitor corrosive activity.
There also remains a need to provide a double bottom apparatus capable of transmitting force from fluid in the tank to the underlying ground while maintaining flexibility.